1. Field of the Invention
This invention relates to a bus line traffic control adapter apparatus, and more particularly to an adapter apparatus for controlling the traffic on a bus line for a terminal apparatus of an ISDN (Integrated Services Digital Network) which performs packet communications.
2. Description of the Related Art
Conventionally, traffic control in packet communications is all performed by an exchange itself in which packet terminals are accommodated, and only involves throughput control of packets in units of a logical channel. Meanwhile, on a bus line of an ISDN, only competition control of the layer 1 level is performed, and securing of the throughput after connection of a line relies upon operation of data terminals connected to the bus line as disclosed in, for example, Japanese Patent Laid-Open Application No. Heisei 2-119340.
FIG. 5 is a diagrammatic view showing a construction of a system of a conventional packet communications network of an ISDN on which traffic control is performed. Referring to FIG. 5, an ISDN exchange 1 is connected to a bus line 12 by way of a digital service unit (DSU) 2. Also a pair of data terminals 10 and 11 are shown connected to the bus line 12.
In a packet communications network of an ISDN, packet transfer is performed either using the B channel or using the D channel for signals. If it is assumed that the D channel is used to perform packet transfer in the system of FIG. 5, although the transmission capacity on the bus line 12 is controlled to 16 kbps of the D channel, there 1s a characteristic in that packet communications can be performed simultaneously by a plurality of data terminals such as the data terminals 10 and 11.
Now, while the first data terminal 10 performs packet communications at 10 kbps with the ISDN exchange 1, if the second data terminal 11 tries to perform packet communications at 10 kbps, since the the first data terminal 10 is using the bus line 12 at 10 kbps, the second data terminal 11 can perform packet communications only at 6 kbps on the bus line 12 of the transfer capacity of 16 kbps.
If this condition continues for a certain period of time and then the transfer rate of the packet communications of the first data terminal 10 drops to 5 kbps, then the second data terminal 11 can perform packet communications at the desired transfer rate of 10 kbps. Then, if the first data terminal 10 now tries to raise the throughput to 10 kbps after the packet communications conditions at the transfer rate of 5 kbps by the first data terminal 10 and at the transfer rate of 10 kbps by the second data terminal 11 continue for a certain period of time, then since the second data terminal 11 is communicating at the transfer rate of 10 kbps, the first data terminal 10 can perform its packet communications only at 6 kbps.
Since the conventional traffic control employs a throughput class controlling method for each logical channel in this manner, the allowable throughput on the bus line 12 is competitively shared between the data terminals, and whether or not a desired throughput can be realized depends upon the communication condition of another data terminal or terminals and a desired communications throughput is not secured. Thus, the conventional traffic control is disadvantageous in that, if the total throughput value of logical channels becomes high, a drop of the throughput occurs, which may give rise to a trouble in communications operation.